I already started a thread just about this
a while back, but it seems to me that there is
a conflict between offering free services to everyone within a geographical area and letting just anyone into that geographical area, a conflict very similar to the "no borders on your bank account" concept Jose mentioned.
Thesh's point about (more or less) that problem going away if all countries were of comparable quality, so people wouldn't be fleeing hellish situations for greener pastures, is true, but only counterfactually so. If
that were the case then
the problem would go away, and we should be working toward making that the case, but in the mean time it's not the case, so while that situation lasts what do we do?
I want open borders. I also want generous public services. If every country everywhere had open borders and generous public services those two things could work out great. But if we're the only ones with both of those things, and everywhere else is either the hellscape other people are fleeing from or else closes their doors to those fleeing, then that puts an undue burden on us.
It's like, I want everyone to have a place to live, I don't want anyone to be homeless, but until we've got some kind of truly egalitarian situation, I don't want to be the only guy in town opening his doors for all the homeless people to come live with me. I want to have my own place to live too, that's not full of otherwise-would-be-homeless people. They definitely deserve a place to live, and I feel sorry for them, but given that I've got to fight so hard just to hold on to my own place to live and keep it in order, why should the burden be just on me to take them in? National borders are basically that, on a larger scale. If the whole world worked like it should, we wouldn't need them and shouldn't have them, but in a world that doesn't work like it should, they're a necessary evil.
And that leaves us with the question of how best to manage them. To which I don't really have any good answers, hence that thread I linked at the start wherein I invite discussion of what immigration policy I should support, since I've kinda got no idea.